Bimetal actuator with electrical resistance heater

ABSTRACT

A bimetal actuator has a thin, flexible, electrical resistance heater lying flat against and bonded to the high expansion side of the bimetal element. The heater comprises a thin, flat, metal resistance element of serpentine form bonded to one side of a thin, flexible, high temperature resistant, plastic film, the other side of the film being bonded to the bimetal. Elongated low resistance portions of the heater extend from the bimetal element for connection to power supply means. In a second form, an additional thin, flexible, high temperature resistant, plastic film overlies and is bonded to the metal resistance element and exposed surfaces of the underlying film and bimetal.

United States Patent n91 Hierholzer, Jr. et al.

[ BIMETAL ACTUATOR WITH ELECTRICAL RESISTANCE HEATER [75] Inventors: Frank J. Hierholzer, Jr., Floriss'ant;

John Ancona, St. Louis, both of Nov. 5, 1974 Primary Examiner-J, D. Miller Assistant Examiner-Fred E. Bell Attorney, Agent, or Firm-Charles E. Markham [5 7] ABSTRACT A bimetal actuator has a thin, flexible, electrical resistance heater lying flat against and bonded to the high expansion side of the bimetal element. The heater comprises a thin, flat, metal resistance element of serpentine form bonded to one side of a thin, flexible, high temperature resistant, plastic film, the other side of the film being bonded to the bimetal. Elongated low resistance portions of the heater extend from the bimetal element for connection to power supply means.

In a second form, an additional thin, flexible, high temperature resistant, plastic film overlies and is bonded to the metal resistance elementand exposed surfaces of the underlying film and bimetal.

5 Claims, 5 Drawing Figures FIG-5 1 BIMETAL ACTUATOR WITH ELECTRICAL RESISTANCE HEATER This invention relates to bimetal actuators having electrical resistance heating means and particularly to I improved electricaL resistance heating means therefor.

BACKGROUND OF INVENTION wire, and, finally, connect circuit leads to the ends of the resistance wire. Usually the resistance wire is a nickel-chrome alloy wire to which low temperature solder does not adhere very well, so that frequently small clamps are employed to secure the ends of the wire and the circuit leads are soldered to the clamps.

It will be readily appreciated that the operations involved in this construction are not well adapted to automation and when performed manually, as they have been, are quite tedious and costly. This is particularly so when the bimetal actuators are quite small and the resistance heater wire is hairlike.

r The rate and uniformity of heattransfer from the resistance heater to the bimetal element are quite critical in arrangements wherein the bimetal actuator functions to operate a switch within a precisely predetermined time following energization of the resistance heater, as in a safety timeswitch. Also, in thermal relays in which the bimetal actuator is operative to cycle a switch on and off at variable predetermined frequencies, as in an electric cooking range control, the cooling time as well as the heating time of the bimetal element is critical,

vand it is. therefore, essential that the thermal mass of the heating means be minimized.

OBJECTS or THE INVENTION ;An' object of the invention is to provide a generally new and improved bimetal actuator comprising a bimetal element and associated electrical resistance heater which isparticularly economical to construct and in which therate of heat transfer between the prising a bimetal element and electrical resistance heating means connected to and overlying onlythe high expansion side of the bimetalelement and flexing with the bimetal element.

A further object is to provide a bimetal actuator as in the preceding paragraph, in which the electrical resistance heating means includes integral flexible conductor portions extending therefrom for connection to terminals or circuit leads.

More specifically, it is an object to provide a bimetal actuator comprising a bimetal element and an electrical resistance heater .therefor consisting of a-thin film of insulating material bonded to the high temperature side of the bimetal element and a thin, flat, flexible, resistance heating element of serpentine form bonded to the film.

. 2 V A further object is to provide a bimetal actuator, as in the preceding paragraph, which includes a second,

thin, flexible film overlying and bonded to the electrical resistance heating element and exposed areas of the underlying film.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is'a plan view of a cantilever mounted, striptype, bimetal actuator constructed and arranged in accordance with the present invention and shown diagrammatically in operative associationwith a control switch and circuit means supplying energy to the resistance heating means;

FIG. 2 is a side elevational view of the bimetal actuator and control switch shown in FIG. 1;

FIG. 3 is an enlarged detailed view of the electrical resistance heating means;

FIG. 4 is a top plan view of a clicker disc-type bimetal actuator shown in operative association with a control switch; and

FIG. 5'is a cross-sectional .view taken along line 5 -5 of FIG. 4. I 1

DESCRIPTION OF DEVICE SHOWN IN FIGS. 1 T

Referring to FIGS. 1 to 3 of thedrawing, a flat, elongated, bimetal element is fixed at one end to a bracket 12. The free end of element 10 overlies the operating pin 14 of a biased closed conventional microswitch 16, which switch is also fixed on bracket 12. The

bimetal element 10 consists of a high expansion strip 18 bonded to a low expansion strip 20. Lying flat against for some predetermined period of time is indicated at 34. The load 34 is connected across terminals 24 in parallel with heater 22 and in series with switch 16 by leads 26, 36 38, 30, and 32. A line switch 35 controls the entire circuit.

Referring to FIG. 3, the electrical resistance heater 22 comprises a thin flexible film 40 and a thin flexible metallic resistance element 42 bonded to the film 40.

The metallic resistance element 42 comprises an interwith minimum stressing. The integral conductor and lead portions obviate the-necessity of soldering or otherwise connecting lead wires to the ends of the high resistance portion 44, which in some instances will be very narrow, thin, and fragile.

The thin flexible film 40 may be of any suitable material, such as polyimide synthetic plastic, having the reper, stainless steel, nichrome, or any other metal suitable for the purpose, and bonded to the film 40 by any suitable means as with a high temperature adhesive such as nitrile rubber. Bonding of the film 40 to the bimetal element may also be accomplished by any suitable means, as with a suitable high temperature resistant adhesive. It is to be understood that the flexible film 40 may be in the order of 0.001 of an inch thick, and the metallic resistance heating element may be in the order of 0.0005 to 0.001 of an inch thick.

The application of a second thin film of suitable material overlying the resistance element 42 and bonded to the resistance element, the exposed areas of the underlying film 40, and the bimetal may be desirable under some conditions. It has been found that the resistance heater may be operated at higher temperatures and under conditions of greater flexing of the bimetal element without separation thereof from the underlying film 40 when a suitable overlying film is so applied. Moreover, the addition of such overlying film provides a protective covering for metal resistance elements against corrosion.

All operations to complete the construction and the attachment of the resistance heater 22 to the bimetal elementIO may be readily automated. For example, in constructing the resistance heater, thin strips of suitable metal may be bonded to thin strips of suitable insulating film to form a continuous laminated tape of suitable width. The configuration of the heating elements 42 shown, or any other desired configuration thereof,

' may be formed by masking and acidetching. Masking be applied as a sheet and bonded to the surface of the resistance heating element and the exposed'surface areas of the underlying film. Also, such overlying film may be applied in a dissolved form by spraying.

OPERATION OF THE DEVICE OF FIGS. 1 TO 3.

When bimetal element is cool, the biased closed switch 16 isin a closed contact position. Under these conditions, when line switch 35 is closed, the resistance heater 22 and the load 34 are connected in parallel across the power source terminals 24 through closedswitch 16. Heating element 44 now heats and heats bimetal element 10, causing it to warp. This causes the free end of the bimetal element to move downward and push open the contacts of switch 16' in a predetermined time. Opening of switch 16 de-energizes the resistance heater 22 and the load 34. The resistance heater now cools and the bimetal element now cools sufficiently in a predetermined time to permit the biased closed switch 16 to again close its contacts.

The time each cycle during which the load 34 will be energized is the time required for the resistance heater to heat the bimetal element 10 sufficiently to overcome the heating element and bimetal element to cool sufficiently to permit the switch biasing means to close the switch. Circuit arrangements, as shown in FIG. 1, may be employed to control the on" time of the heating element of an electric blanket or the top heating elements of an electric cooking range. In these uses of the device, the period of on time each cycle may be varied as desired by varying the closing bias of switch 16 or by other adjustable means variably opposing the downward force of the free end of bimetal element 10 when it is heated.

In FIGS. 4 and 5, the round, bimetal, clicker disc 10a is retained at its periphery at the rim of a cup-like casing 12a. The disc 10a-has a high expansion element 18 and a low expansion element 20 bonded thereto. At normal temperatures, the disc 10a assumes an outwardly concave form, as shown. In this form, the convex side thereof engages an operating p'in 14a of a biased open switch 16a, thereby holding the switch in a closed position against its opening bias. Lying flat against the concave high expansion side 18a of the element and bonded thereto is. a electrical resistance heater 22a. The resistance heater 22a while having a different peripheral configuration than that of heater 22, shown in FIG. 3, may otherwise be constructed'similar thereto, including integral conductor and lead portions.

When resistance heater 22a is energized, it heats the high expansion side 18 of disc 10a, which causes the disc to snap through a planar form to an outwardly conthe closing bias of the switch, and the timeeach cycle during which the load will be off is the time required for by its biasing means. The device shown in FIGS. 4 and 5 may be employed tope'rform a safety cutoff function. In such use, the resistance heater is connected across a power supply through temperature responsive means, such as a thermistor, positioned 50 as to respond to a hazardous high temperature condition and permit current to flow through heater 22a.

Bimetal actuators, in accorda'nce with the foregoing disclosure, comprising a bimetal element arranged to actuate a switch and having a flexible resistanceheater bonded to the high expansion side thereof have been constructed by applicants. By way of example, in one construction of the device, a 0.0005 inch thick strip-of stainless steel foil was bonded to a 0.001 inch thick strip of polyimide film with a nitrile rubber-phenolic adhesive. Employing the photoresist method, the stainless steel foil was then masked and acid etched to achieve a configuration of the resistance element similar to that shown in FIG. 3 of the drawing. The'heater was then cut out from the film strip and bonded to the high expansion side of the bimetal element, again with.

high temperature, it was found that the device could be operated at considerably higher temperature without delamination of the resistance element from the film if an additional plastic film in the orderof 0.001 inch thick, overlying and bonded to the resistance element and exposed surface of the underlying film, is provided. Also, by extending such overlying'filmto include adjacent areas of the bimetal surface,'additional b'ondingof the entire heater to the bimetal is provided.

We claim:

l. A bimetal actuator comprising a bimetal element with support means fixing a portion thereof so that another portion moves relative to the support means when the element warps and a thin flexible resistance heater bonded to the high expansion side of said element, said heater comprising a thin, flexible, insulating film and a thin, narrow, flexible, metallic, electrical resistance element bonded to one side thereof, the other side of said film being bonded to the high expansion side of said bimetallic element, said insulating film including portions thereof extending freely from said bimetal element, and said resistance element including integral lead portions at the ends thereof, which lead portions are considerably wider than said resistance element and coextend with and are bonded to said freely extending film portions thereby to provide integral flexible leads.

2. The bimetal actuator claimed in claim 1 in which said electrical resistance element further includes integral conductor portions between the ends thereof and said integral lead portions, which conductor portions are wider than said resistance element and narrower than said lead portions and in which said conductor portions overlie and are bonded to those portions of a second thin insulating film overlying and bonded to end portions thereof extending laterally from said bimetal strip, said electrical resistance element having a highiresistance portion comprising a thin, long, relatively narrow, metallic strip lying in a generally serpentine pattern with elongated transversely extending and closely spaced parallel portions joined by short portions, and relatively wide integral end portions extending laterally from said bimetal strip and bonded to said laterally extending film portions, thereby to provide integral flexible leads of relatively low resistance.

5. The bimetal actuator claimed in claim 4 in which said insulating film is a high temperature resistant plastic film and in which said film is bonded to said bimetal element with a high temperature resistant adhesive. 

1. A bimetal actuator comprising a bimetal element with support means fixing a portion thereof so that another portion moves relative to the support means when the element warps and a thin flexible resistance heater bonded to the high expansion side of said element, said heater comprising a thin, flexible, insulating film and a thin, narrow, flexible, metallic, electrical resistance element bonded to one side thereof, the other side of said film being bonded to the high expansion side of said bimetallic element, said insulating film including portions thereof extending freely from said bimetal element, and said resistance element including integral lead portions at the ends thereof, which lead portions are considerably wider than said resistance element and coextend with and are bonded to said freely extending film portions thereby to provide integral flexible leads.
 2. The bimetal actuator claimed in claim 1 in which said electrical resistance element further includes integral conductor portions between the ends thereof and said integral lead portions, which conductor portions are wider than said resistance element and narrower than said lead portions and in which said conductor portions overlie and are bonded to those portions of said film which are bonded to said bimetal element.
 3. The bimetal actuator claimed in claim 1 including a second thin insulating film overlying and bonded to said resistance heating element, to the exposed surfaces of said insulating film, and to Adjacent surfaces of said bimetal element.
 4. A bimetal actuator comprising an elongated bimetal strip having a thin, flexible, electrical resistance heater bonded to the high expansion side thereof, said heater comprising a thin elongated strip of insulating film lying longitudinally along the high expansion side of said bimetal strip and a thin, flexible, metallic, electrical resistance element bonded to one side of said film, the other side of said film being bonded to said bimetal element, said flexible insulating film including end portions thereof extending laterally from said bimetal strip, said electrical resistance element having a high resistance portion comprising a thin, long, relatively narrow, metallic strip lying in a generally serpentine pattern with elongated transversely extending and closely spaced parallel portions joined by short portions, and relatively wide integral end portions extending laterally from said bimetal strip and bonded to said laterally extending film portions, thereby to provide integral flexible leads of relatively low resistance.
 5. The bimetal actuator claimed in claim 4 in which said insulating film is a high temperature resistant plastic film and in which said film is bonded to said bimetal element with a high temperature resistant adhesive. 